In the field of gears for use in automobiles and construction machines, there has recently been an increasing demand for cost reductions by less processing time and for improved surface pressure bearing strength intended for manufacture of compact power transmission systems. With a view to reducing processing time, high precision cold forging is under study, because where blank material is produced by conventional hot forging, dimensional tolerance is so poor that a lot of cutting work is required in the subsequent process of machining. For improving surface pressure bearing strength, there have been made several attempts which include positive addition of Mo element with the intention of improving the resistance of steel to softening caused by tempering. Another attempt is the method in which a material is quenched after carburization and carbonitriding and then subjected to shot peening, whereby the hardness of the surface layer is increased and noticeable compressive residual stress is imparted to the material.
In the method in which the teeth of a gear is formed by hot forging, when .gamma.-phase steel (austenitic steel) heated to 1,200 to 1,300.degree. C. is upset within a forging die at room temperature, the heated steel is rapidly cooled, causing a rapid increase in resistance to deformation, which imposes significant stress on the die or causes significant amount of wear in the die during the formation of elaborate gear teeth. Therefore, the die should be sufficiently rounded to provide an elaborated gear shape and the temperature of the die should be markedly increased to constrain cooling of the blank when contacting with the die. Under such a situation, it is difficult to produce high precision forged blanks for gears. Although it is conceivable that forging speed is increased so that the material blank can be prevented from cooling by the shearing heat of the forging material, this leads to a further increase in the deformation resistance of the material, which arises the need for bigger round die portions and, in consequence, more problems in high precision forging.
More compact gears have smaller tooth profiles and blanks for such gears are more easily cooled, so that the above-described problem becomes more noticeable.
An attempt has been made to form high precision gear teeth by cold forming by use of hot forging material, which however involves two stages, entailing a significant increase in cost.
In the hot forging process described above, since the gear material is once heated to 1,200 to 1,300.degree. C., the crystal grains having the austenitic phase become extremely coarse, and this brings about a significant difference in deformation resistance between the rapidly cooled parts and other parts of the forming material. As a result, there remains irregular processing distortion in the gears. To avoid the distortion of the gears caused by machining and carburization as far as possible, sphering and distortion removal is carried out by cooling or normalizing etc. prior to machining in most cases. This also increases cost.
Warm forging has been proposed taking the above problems into account, in which the steel material is heated to 850 to 1,000.degree. C. which is lower than hot forging temperatures and deformation resistance is reduced with the help of the .alpha. phase in order to quickly perform high precision forging while the steel is in the (.alpha.+.gamma.)-Fe two phase structure region in the course of the forging operation. However, this method also suffers from the problem that since a heavy deformation process is involved when the .alpha. phase precipitates from the .gamma. phase crystal boundary, boundary exfoliation often occurs within the matrix so that the material is likely to be brittle.
To follow the recent trend toward the production of high-power, light-weight and compact reducers and transmissions, improved surface pressure bearing strength is required especially in gears. As explained above, gears are generally manufactured by applying surface heat treatment such as carburization and carbonitriding to material after machining to harden their surface layers and designed so as to withstand high contact pressure (Hertz's surface pressure). Usually, such heat treatment takes long time increasing the production cost of gears. Reduction gears for construction machines have large-sized modules in many cases and RX gas carburization for such gears normally takes a couple of days. Therefore, various methods using high carburization temperatures are now under study. Introduction of high carburization temperatures in RX gas carburization, however, encounters difficulty in controlling the carbon potential of the carburization so as to maintain a CO/CO.sub.2 gas equilibrium condition. For instance, in the carburization phase with high carbon potential, coarse cementite precipitates on the surface of the gear material, leading to a decrease in gear strength. With view to preventing the precipitation of cementite, a diffusion process is carried out for a length of time equal to the time required for the carburization phase or more, thereby adequately adjusting surface carbon concentration. This measure, however, cannot overcome the above problem, i.e., the difficulty in performing high-accuracy carbon potential control.
For producing gears capable of withstanding higher contact pressure to follow the aforesaid recent trend, appropriate alloy composition is sought by addition of Mo and/or V to steel material, which increases resistance to softening caused by tempering in the surface hardened layer obtained after quenching, or by addition of Nb and/or Ti to steel material, which makes the crystal grains finer. High-power shot peening is adapted to further harden the surface hardened layer. However, in spite of all efforts, none of the above measures has turned out to be effective.
Positive addition of the elements that form a fine special carbide in austenite such as V, Nb and Ti with view to reinforcement of gears leads to a considerable increase in the deformation resistance of the austenite at high temperatures and therefore such alloy designs are not suitable when taking the above-mentioned plasticity workability into account.
The present invention is directed to overcoming the foregoing problems. Therefore, one of the objects of the invention is to provide a steel material with which deformation resistance generally occurring in plastic working can be reduced and stable high precision plastic working is enabled at lower temperatures, when producing toothed material for high strength gears etc. by simple plastic working instead of machining. Another object of the invention is to provide a method for producing a rolling element such as gears having high surface pressure bearing strength by applying surface heat treatment such as carburization and carbonitriding to the above steel material.